Follow the steps to "Get started" with your Cornerstone shave plan - you will be able to enter the promotional code at the end of the process.
Before hitting the final "join now" button you should find a link stating "If you have a gift voucher please click here". Clicking it will open a box and you can enter your code there. Be sure to then select "Apply voucher" before proceeding with finalising your order.
If you have followed a friend's referral link or followed a promotional link the discount should have applied automatically to your first order.